Based on the feedback here and what I saw in the demo, I went ahead and popped for Archivarius about a week ago. It was only $29; I figured if it didn't work out I could give the thing to my sister as a gift. My plan was to keep both Archivarius and X1 running side-by side for awhile to contrast and compare.
Archivarius took about 8 hours to build its initial index, after a few false starts. When I noticed it was opening .ISO images and indexing their contents I stopped the index operation and excluded them. This happened a few times as I spotted Archivarius getting hung up on various large media I did not want indexed.
By contrast X1's reindexing is multi-threaded and automatic. X1 allows use of the search tool while indexing is in progress, Archivarius does not.
I wonder, however, how much horsepower is sucked up with X1's background indexer running all the time. I've watched its indexing process + document unpacker with Process Explorer, and caught it using horrific amounts of RAM and CPU for brief times. But I cannot actually say it's had an impact on my user experience. I suspect, however, that it has contributed to some system instability since X1's document unpacker gets hung occasionally.
Big plus: Archivarius indexed two network drives I specified -- something X1 has trouble with. X1 *says* it indexes my network shares, but search terms targeted at network media return zilch.
Reindexing completed, the first test I ran was to look for the same search terms in both products. Archivarius generally returned results slower than X1, but X1 does that "autocomplete" thing which narrows the search as you type. Personally, I never liked that, as it messes with my mind while I'm trying to type. I much prefer Archivarius' old school "collect the parameters then search" approach. So point to Archivarius.
Search results are much prettier in X1, a more finished/polished display. But for my purposes, I'd rather view the document using its original container/app, so that has never impressed me. I want search engines to search, dammit, and leave the rest of it to the operating system. Archivarius' results pane is very usable, even if some of my email is formatted funny. But again -- I want fast, accurate results and in this Archivarius delivers.
And accuracy: Archivarius seems to do a MUCH better job at finding things than X1. For instance, I *know* have a Word document someplace with some old bank account numbers, long since closed. I've searched for it many times with X1 and always came up dry. Archivarius found it on the first try, and ranked it #1.
Another example: some email from a science-fiction author I'd communicated with back in 2004. I wanted to drop him a line and say hi again, but could not find his email address with X1. Archivarius found it, again, first time out and ranked it #1. It also found the entire email thread from 2004, our entire exchange, as a bonus. I thought it had been lost forever, victim perhaps of the great Format C: DrivePocalypse of 2005.
I was so impressed I uninstalled X1 right then and there. Archivarius is now my primary desktop search tool. This is a big deal for me, since I was an X1 early adoptor, having paid $79 for the thing when it was spanking new, before it became the basis of Yahoo Desktop search. I've participated in their betas, installed crappy test builds, and worked with their tech folk to resolve issues which later got folded into future builds. For awhile, I felt invested in the product and the company. Since then I've drifted away and haven't even been downloading the latest X1 releases, but I still felt strongly about the product.
So ditching X1 in favor of Archivarius is high praise indeed coming from someone like me.
Thanks to everyone who suggested Archivarius! It's a great program.
My system specs: Dell Optiplex GX620, 4GB RAM, 1.3TB disk space, XP SP2.
Archivarius stats: 5.59 GB, 1.07 milliion documents, 369 GB files indexed. Time to build index, 7h 48m.